Heater concentricity in the extrusions of thermoplastic to make tubular film



Jan. 25, 1966 M. GOLDMAN ETAL 3,231,643 HEATER CONCENTRICITY IN THE EXTRUSIONS OF THERMOPLASTIC TO MAKE TUBULAR FILM Filed July 9, 1964 COOLANT IN COOLANT 22 our INVENTORS MAX GOLDMAN MIKLOS WALLENFELS ATTORNEY United States Patent Ofi ice 3,231,643 Patented Jan. 25, 1966 3,231,643 HEATER CONCENTRICITY IN THE EXTRUSIONS F THERMOPLASTIC TO MAKE TUBULAR FILM Max Goldman, Tonawanda, and Miklos Wallenfels, Buffalo, N.Y., assignors to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware Filed July 9, 1964, Ser. No. 381,470 The portion of the term of the patent subsequent to July 21, 1981, has been disclaimed 12 Claims. (Cl. 26425) This application is a continuation-in-part of copending application Serial No. 51,629, filed August 24, 1960, and now US. Patent No. 3,141,912.

This invention relates to the orientation of thermoplastic polymeric film in tubular form. More particularly, it relates to a method for orienting by a combination of expansion and longitudinal stretching to improve the properties of thermoplastic, polymeric, tubular film.

The idea of orienting under controlled conditions, particularly at a controlled temperature, to improve the physical properties of thermoplastic polymeric film, is not new. The idea of expanding a thermoplastic, polymeric, tubular film by providing gas under pressure within the tubular film is not new either. The problem, however, of uniform heat application and, thus, maintaining control over the temperature of the tubular film prior to and during expansion of the film has also prevailed for a long time and is a problem that has discouraged the use of the expansion or blowing process for the orientation of polymeric film. There are two factors which hinder this accomplishment: (1) non-uniformity of gauge of the tubular film, and (2) eccentricity of the heating and cooling sources with the tubular film.

It is, therefore, an object of this invention to provide a process for maintaining temperature uniformity of thermoplastic, polymeric tubular film during expansion to provide a process for orienting such film.

It is a further object of this invention to provide a process for the biaxial orientation of thermoplastic, polymeric tubular film in which the tubular film is maintained concentric to the heating sources. These and other objects will appear hereinafter.

The manner in which the objects of the invention are attained is set out in the following description and the drawing, in which:

The figure is a schematic view, partially in section, of an apparatus adapted to carry out the process of the present invention.

The objects are accomplished by a process which involves, first extruding thermoplastic polymeric material in the form of a tubular sheet or film heated to thermoplastic condition, i.e., in a formative state above its crystalline melting point; advancing the tubular sheet at a predetermined initial rate; quenching the tubular sheet by cooling the sheet quickly to a temperature below the formative state, preferably at least 20 C. below the orientation temperature range; maintaining sufiicient pressure within the tubular sheet to prevent collapse and to provide, when the tubular sheet is heated to a temperature within its orientation temperature range, an expansion of at least 2 times the original diameter of the tubular sheet; heating the tubular sheet to a temperature within the orientation temperature range of the polymeric material to expand the tubular sheet to a diameter at least 2 times, preferably 210 times its original diameter as extruded; maintaining the tubular sheet concentrically with the heat source, preferably, heating the tubular sheet on its exterior and interior surfaces from spaced apart radiant heat sources located concentrically around the periphery and concentrically inside the tubular sheet to a temperature within the orientation temperature range to expand the tubular sheet to a diameter at least 2 times its original diameter, most of the heat supplied by the external heater, said expansion being carried out while said tubular sheet is exposed to said internal heater; advancing while expanding the heated tubular sheet at a rate at least 2 times, preferably 210 times the initial rate, preferably the ratio of the final advancing rate to the intitial advancing rate being equal to the ratio of the final diameter to the extruded diameter of the tubular sheet; and, finally, cooling the tubular sheet while maintaining the sheet substantially at its expanded diameter.

The orientation temperature range, as defined herein, refers to the temperature range in which molecular orientation of a polymeric film may be effected. This range lies somewhere below the melting temperature of a polymer that melts at a specific temperature or below the crystalline melting point of a crystalling polymer that melts over a range of temperatures. The crystalline melting point refers to the temperature at which the crystallites of a crystalline polymer are no longer detectable under X-ray examination when the solid polymer is heated until it melts.

For some crystalline polymers such as polyethylene, polypropylene and other polyhydrocarbons, the orientation temperature range may be the range of temperature over which the crystallites melt but below the temperature at which the crystallites are no longer detectable. In the case of polyesters such as polyethylene terephthalate and the like, the so-called crystallizable polymers, the orientation temperature range extends from about 10 C. to 40 C. above the second order transition temperature of the polymer. The second order transition temperature is that temperature at which an essentially amorphous polymer or one that can be quenched as an amorphous polymer but is crystallizable makes a transition from a glassy state to a rubbery state. It is in this rubbery state that the polymer in the form of a film or a filament can be oriented by stretching. The second order transition temperature varies with the molecular weight of the polymer and is defined more completely in US. Patent 2,578,899.

The specific orientation .temperature range will vary from polymer to polymer but may be determined by experimentation or from the literature. In the following table, Table I, the orientation temperature range, the second order transition temperature and the crystalline melting point are given for some representative amorphous polymers and some crystallizable polymers that are amorphous as quenched.

TABLE I Second Order Crystalline Orientation Polymer Transition Melting Temperature Temp. C 0.) Point 0.) Range C.)

Polyethylene terephthalate 70 255 85-110 Polyethylene-2,G-naphthalate 113 265 120-140 Polytetramethylene-l,2 dioxybenzoate 53 220 70-90 Polyethylene-1,5-naphthalate 71 225 80-100 Polyhexamethylene adipamide- 45-50 250 65-75 Polyhexamethylene sebacamida. 45-50 250 65-75 Polyeaprolaetam 45-50 250 65-75 70% Ethylene terephthalate/30% ethylene isophthalate co olymer 51 170 70-90 Polyvinyl chlori e:

N o plasticizer- 105 170 115-145 plasticizer- 90 170 100-130 platicizer- 75 170 85-115 plasticizen- 60 170 70-100 84% tetrafluoroethylene/l6% hexafluoropropylene copolymer 85 275 95-125 Polystyrene None *88-120 88-110 Polymethylmethacrylate None *66-111 66-105 *So ftening range rather than crystalline melting point since the polymer is only obtainable as an amorphous polymer.

In the following table, Table II, the orientation temperature range and the crystalline melting point are listed for some representative crystalline polymers.

In the following table, Table III, the preferred conditions of this process for several particularly important polymers are listed.

ing disc 15. The disc has openings 26 around its periphery, the openings communicating with the atmosphere through conduit 27. Coils 30 communicating with inlet and outlet tubes 28 and 29 in the conduit, are disposed adjacent to the interior surface of the mandrel. The coils 30 convey cooling water which serves to reduce the temperature of the tubular film to about room temperature, although any temperature from 10 C. to about C. would be adequate to convert most thermoplastic polymeric films to a non-formative plastic state, i.e., a state where the film resists stretching. Since cooling tends to shrink the thermoplastic polymeric tubular film, the mandrel is preferably tapered to the contour of the shrinking film. If necessary, pressure relief areas such as those disclosed in U.S. Patent 2,987,765 may be provided to isolate the gas pressure downstream of the mandrel 12.

The solidified tubular film is then drawn through a guide ring 16, the ring serving to minimize sway of the tube. Drawing is accomplished by means of a set of nip rolls 24 rotating at a rate that is at least 2 times the rate 1 Density above 0.909 gmsJcc.

2 Density above 0.92 gms./cc.

8 Plastieizer content of 015%.

Referring to the figure, which is a diagrammatic illustration of the process of this invention, the thermoplastic polymer is first heated to a temperature above its melting point in the extruder 10. The molten polymer, preferably at a temperature at least 10 C. above its melting point or crystalline melting point, is extruded through die 11 in the form of a tubular film 14, the tube having a wall thickness of anywhere from 15 to 85 mils. The tubular film is then drawn over a cooling mandrel 12 by means of the tube advancer 13. The tube advancer 13 is composed of two sets of driven rubber squeeze rolls which contact the surface of the cooled tubular film to advance the film at a predetermined initial rate. The tube advancer 13 also serves to prevent sway of the film and to insulate the freshly extruded film adjacent the die from the subsequently applied longitudinal tension. Instead of driven rolls, endless belts may be used as the tube advancer.

The cooling or quench mandrel 12 is a hollow metal cylinder insulated from the die by a non-metallic insulatof the tube advancer 13. Next, the film is advanced through the initial heating section 19 concentrically positioned around the periphery of the tubular film. This section is composed of the forced air heaters 17 and the radiant heater 18 connected to a power source, not shown. The connecting wires are also brought in through the conduit 27. It should be understood that any other means that would serve to heat the outside surface of the tubular film would operate in this invention. The initial heating section 19 serves to heat the film to a temperature within about of the lowest temperature of the orientation temperature range to just below the orientation temperature range of the thermoplastic polymer. The precise temperature to which the film is heated in the initial heating section 19 will depend upon several factors. For example, the greater the speed of the film, the higher the temperature to which the film must be heated. The greater the intensity of the final-internal heater 22, the lower the temperature to which the film need be heated in the initial heating section 19.

Air or other gaseous medium admitted through inlet 20 and vented through outlet 21 provides the pressure within the tubular film to prevent collapse of the film while the film is at a temperature below the orientation temperature range and to expand the tubular film a predetermined amount when the temperature of the film reaches the orientation temperature range. This circulating air also serves to cool the inside surfiace of the tubular film. The amount of expansion is dependent upon the pressure within the film, the precise temperature within the orientation temperature range to which the film is brought, the rate of heating the film, the thickness of the film wall, etc., and can either be pre-set by one skilled in the art or set after experimentation.

The tubular film is brought to a temperature within the orientation temperature range and at which the film expands by means of the pencil-type, internal, centrally located radiant heater 22 mounted on the internal guide ring 23. A disc-type internal radiant heater can also be used. Passing tubular film 14 over internal guide ring 23 maintains the tubular film in strict concentricity with heater 22. Heater 22 is connected to a power source by wires leading through the conduit 27 to a power source, not shown. The tubular film 14, immediately upon reaching the orientation temperature range, starts to expand due to the pressure within it and to elongate due to the relative rates of the nip rolls 24 land the tube advancer 13. However, as the tubular film expands, its inner surface gets farther and farther away from heater 22 and its outside surface approaches the cooling ring 25. This combination of factors serves to reduce the temperature of the film while stretching is occurring. The resulting expanded and stretched tubular film is then cooled by continuing its passage through the cooling ring 25. Cooling ring 25 can contain coils to convey cooling water in its surface as in the quenching mandrel. It should be understood that neither the precise heating means nor the precise cooling means described or shown in the figure are to be construed as limitative in the present invention.

To illustrate the important effect of tubing eccentricity with the internal heater, the following table shows the temperature difference for a film at specified distances from the centerline of an internal radiant heater for an assumed control condition.

It is readily apparent that a small increase or decrease in the distance from the internal heater has a great effect on the temperature of the film. It is obvious that the same considerations for concentricity and deviations therefrom applies for an external heating source.

The need for the uniformity of radiant heat intensity on the tubular film emphasizes the importance of the tubing concentricity and, thus, makes guiding of the tube a prime consideration. Many effective internal and external guide devices can be employed, the most advantageous guides being those fabricated from materials which will provide the least drag force on the film surface should the tube make intermittent contact. Such guide means, other than the internal guide ring shown in the drawing, can include an external tetrafiuoroethylene polymer ring or an internal tetrafiuoroethylene polymer disc. An air bearing is also an effective guide using air of the bubble pressure or from an external source to provide lubrication between the film and guide.

Most of the heat to the tubular film is supplied from the first heater thereby making it advisable to maintain a strict concentricity of the tubular film with the first heater. In those cases where the first heater is an external heater, it has been convenient to use an external guide in conjunction with the external heater. Similarly, since it is important to raise the tubular film uniformly to its orientation temperature for expansion under the given internal pressure employing an internal heater, concentricity can be maintained and guiding accomplished through the use of external or internal guides. However, internal guides are more readily adjustable to maintain concentricity between the internal heater and tubular film. Whether using internal or external guides, one should recognize the neck-down characteristics of the polymer employed and design the guides accordingly.

The importance of concentricity and guiding has been demonstrated on the effect of radiant heat intensity of the tubular film. Further, however, is the fact that the radiant heaters themselves do not emit heat uniformly. To permit the tubular film to be more uniformly heated, rotation of the heaters, internal and external, is desired.

At a minimum, some rotation (or contra-rotation or oscillation) is helpful. Preferably, the heaters should rotate as fast as practical so that the tubular film, in fact, would sense an integrated heat intensity. The film, in essence, would not recognize any heater non-uniformities. More practically, the first heater, external, should be rotated at a rate of one revolution per length of unit travel of tubular film through the external heater length.

Since in most tubular extrusion orientation processes the extrusion die is rotated, it is a practical expedient to rotate the heaters in synchronization with the die.

By adhering to the limits of the process of the present invention, several surprising results are at once apparent. The resulting biaxially oriented film does not display the usual magnified gauge variations normally accompanying film-blowing processes. That is, gauge variations, almost unavoidable during extrusion, are not magnified as in prior art blowing processes by the expansion process of the present invention. Another unexpected result is that polypropylene and other crystalline polymers which heretofore had suffered from line drawing when stretched by expanding the tubular film, do not suffer from this fatal shortcoming when subjected to the process of the present invention.

The following examples serve to illustate the present invention, Example 1 being the best mode contemplated for practicing the invention.

Example 1 Molten polypropylene having a density of 0906* was extruded at a temperature of 225 C. and at a rate of 10 pounds per hour from a 1-inch extruder through a 2% inch circular die having a 45-mil lip opening. The apparatus was substantially that shown in the drawing. The extruded tubular film was quenched to the non-formative plastic state by being passed over a nominal 2-inch tapered internal mandrel at a temperature of about 10 C., the taper of the mandrel being 4 mils per inch. The tubular film was withdrawn from the die over the internal mandrel by a tube advancer operating at 2.5 feet per minute. Air was injected and vented to the atmosphere through connections in the end of the quench mandrel. The resulting open bubble system provided a pressure of 22 inches of water within the tubular film. As the tubular film cooled over the internal mandrel, it shrank in diameter. This shrinkage, together with the taper of the mandrel, served to prevent the air injected at the end of the mandrel from exterting pressure on the molten tubular film in the area adjacent the die.

After pasing through the tube advancer, the tubular film was led through an external tapered guide ring 6 inches long and 2 inches in diameter to minimize sway and maintain the tube concentric with the external initial heater. The tubular film was then passed into an initial heating zone where it was heated to a temperature of C. The initial heating zone was composed of several external radiant heaters that surrounded the film Pro-fax 6512 E.

and a forcedair heater. The radiant heaters were each 12 inches high, 10 inches in diameter, rated at 6500 watts and operated at'25% capacity. The forced air heater, which was located just upstream of the radiant heaters, forced -Tensile strength, elongation and initial tensile modulus are normally measured at 23.5 C. and 50% relative humidity, although they may also be measured at other specified temperatures and humidities. They are air at a temperature of about 160 C. and at a rate of determined by elongating the film sample in an Instron about 1,200 feet per minute against the outside surface tensile tester at a rate of 100% per minute until the of the tubular film. sample breaks. The force applied at the break in p.s.i. is

Immediately after passing through the initial heating the tensile strength. The elongation is the percent inzone, the tubular film was passed into the final heating crease in the length of the sample at breakage. Initial zone where it was heated to a temperature of about 155 10 tensile modulus in p.s.i. is directly related to the film C. Heating was provided by a pencil-type internal rastiffness. It is obtained from the slope of the stressdiant heater 7 inches long, rated at 650 watts, operated strain curve at an elongation of 1%. Both tensile at 72% capacity and having a surface temperature of strength and initial tensile modulus are based upon the 620 C. which is mounted on the internal guide ring. At initial cross-sectional area of the sample. the internal pressure of 22 inches of water and at the Pneumatic impact strength or impact strength is the temperature of 155 C., the tubular film stretched and energy required to rupture a film. It is reported in kiloexpanded. Stretching of six times its original length was grams-centimeters per mill thickness of the sample. The accomplished a Set Of p T0118 that drew the film at a pneumatic impact strength is determined by measuring rate of 15 feet per minute. Expans n t a eter the velocity of a ball mechanically accelerated by air of 12 inches took Place due t0 the internal Pressure 0f pressure, first in free flight and then in a flight impeded 22 inches of water at this temperature. by rupturing the test sample. In this test, the film sample The major amount of coolmg w accomphshed by is 1%" x 1%". The projectiles are steel balls /2" in Passing the expanded film through a Water-cooled i diameter and weighing 8.3 grams. The free flight veloc- 101/2 inches long and 121/2 inches in diameter Whlch ity is 23 meters per second. The velocities are measured served to C001 the expanded and Stretched film a 25 by timing photoelectrically the passage of the steel balls Denture about The tubular film Whlch f between two light beams set a definite distance apart. been elongated SiX times in both the transverse and longlpneumatic impact trength is measured the loss 'fl l directions Was y collapsed as It Passed in kinetic energy due to the rupturing of the test sample. through the nip of a set of mp rolls and was finally wound It is calculated fr the foll wi f ul on a I011 Constant S uare of velocity in free flight square of The film having a thickness of 0.8 mil displayed the folcielocity in impeded flight) lowmg proper where the constant is directly proportional to the weight L it 1 T of the projectile and inversely proportional to the acceler- $5 3 3? EZZEQESS? ation due to gravity. This test is carried out at 23 C. LD TD and 50% relative humidity and the test samples are conditioned for 24 hours at 23 C. and 50% relative humidity. l i id ri g ti dr i ai gr ce f%fi?.::ZI: 82 22 Shrinkage is a measure of the form-stability of the Initial Tensile Modulus (1bs./sq.in.) 395,000 340,000 40 1 T Sample Strips, 25" X 5" each f h Impact Strength (kgs.-cm.]mll.) .7 direction, i.e. ten having the longer dimension running h k t 3 9 6 in the machine 017 longitudinal direction or the direction S an age (percen in which the film was extruded and ten having the longer dimension running in a direction transverse to the mafth film h 11th t ii$i$ iriifiiififiha1han s? e appara us chine direction, are supported from two ad acent corners 3 Independent of direction and suspended in boiling water for 1 second. The dimen- TABLE 1V Example Polymer Kopper Esso Super Alathon 14 Polyvinyl (266C)! Dylan 25% A-71X3 Chloride Extrusion Temp. C.) 225 215 215 190 Quench Temp. C C.) 25 25 25 25 Initial Diameter (inches) 2 2 2 2 Initial Rate of Drawing (ft./min.) 2. 6 2. 5 2. 5 2. 5 Temp. of Film in Initial Heating Zone 0.) 125 115 so Surface Temp. of External Quartz Heater 0.) 250 280 320 200 Bubble Pressure (inches of water) 24 8 6 8 Temp. of Film in Final Heating Zone 0.) 150 125 90 Surface Temp. of Internal Radiant Heater 0.) 610 570 350 700 Final Diameter (inches)- 10 12 10 4 Final Rate of Drawing (i m 13 14. 5 12. 5 8 Amount of LD Stretch 5X 6X 5X 3. 2X Amount. of TD Stretcl1 5X 6X 5X 2. 0X

1 Linear polypropylene having a density of 0.902. 1 Linear polyethylene having a density of 0.954

4 A composition containing 92% polyvinyl chloride, 4% lauryl mercaptan and 4% betapinene.

sional change is then noted, and percent shrinkage is calculated based on the original dimension.

Examples 2-5 Example \1 was repeated using different polymeric materials in substantially similar equipment as that shown in Example 1. The particular polymeric materials and the specific operating conditions are summarized in Table IV.

The thickness and the physical properties of the films What is claimed is:

1. The process comprising: extruding thermoplastic polymeric material in the form of a tubular sheet in its formative state; advancing the tubular sheet at a predetermined initial rate; quenching the tubular sheet to a temperature below its formative state; maintaining sufiicient pressure within the tubular sheet to prevent collapse and to provide, when the tubular sheet is heated to a temperature within its orientation temperature range, an expansion to a diameter of at least 2 times the original diameter of the sheet; heating the tubular sheet from a radiant heat source to a temperature Within the orientation temperature range of the polymeric material to expand the tubular sheet to a diameter at least 2 times its original diameter; maintaining the tubular sheet concentrically with the heat source; advancing, while expanding, the heated tubular sheet at a rate at least 2 times the initial rate of advancement; and cooling the tubular sheet while maintaining the sheet substantially at its expanded diameter.

2. The process of claim 1 wherein the radiant heat source is rotated.

3. The process comprising: extruding thermoplastic polymeric material in the form of a tubular sheet in its formative state; advancing the tubular sheet at a predetermined initial rate; quenching the tubular sheet to a temperature below its formative state; maintaining sufficient pressure within the tubular sheet to prevent collapse and to provide, when the tubular sheet is heated to a temperature within its orientation temperature range, an expansion to a diameter of at least 2 times the original diameter of the sheet; heating the tubular sheet on its exterior and interior surfaces from radiant heat sources located concentrically around the periphery and concentrically inside the tubular sheet to a temperature within the orientation temperature range to expand the tubular sheet to a diameter at least 2 times its original diameter, most of the heat supplied by the external heater, said expansion being carried out While said tubular sheet is exposed to said internal heater; advancing, while expanding, the heated tubular sheet at a rate at least 2 times the initial rate of advancement; and cooling the tubular sheet while maintaining the sheet substantially at its expanded diameter.

4. The process comprising: extruding thermoplastic polymeric material in the form of a tubular sheet in its formative state; advancing the tubular sheet at a predetermined initial rate; quenching the tubular sheet to a temperature below its formative state; maintaining suflicient pressure within the tubular sheet to prevent collapse and to provide, when the tubular sheet is heated to a temperature within its orientation temperature range, an ex- 10 pansion to a diameter of at least 2 times the original diameter of the sheet; heating the tubular sheet on its exterior and interior surfaces from spaced apart radiant heat sources located concentrically around the periphery and concentrically inside the tubular sheet to a temperature within the orientation temperature range to expand the tubular sheet to a diameter at least 2 times its original diameter, said expansion being carried out while said tubular sheet is exposed to said internal heater; advancing while expanding, the heated tubular sheet at a rate at least 2 times the initial rate of advancement; and cooling the tubular sheet while maintaining the sheet substantially at its expanded diameter.

5. The process comprising: extruding thermoplastic polymeric material in the form of a tubular sheet in its formative state; advancing the tubular sheet at a predetermined initial rate; quenching the tubular sheet to a temperature below its formative state; maintaining sufficient pressure within the tubular sheet to prevent collapse and to provide, when the tubular sheet is heated to a temperature Within its orientation temperature range, an expansion to a diameter of at least 2 times the original diameter of the sheet; heating the tubular sheet on its exterior and interior surfaces from spaced apart radiant heat sources located concentrically around the periphery and concentrically inside the tubular sheet to a temperature within the orientation temperature range to expand the tubular sheet to a diameter at least 2 times its original diameter, most of the heat supplied by the external heater, said expansion being carried out while said tubular sheet is exposed to said internal heater; advancing, while expanding, the heated tubular sheet at a rate at least 2 times the initial rate of advancement; and cooling the tubular sheet while maintaining the sheet substantially at its expanded diameter.

6. The process of claim 5 wherein the radiant heat sources are rotated.

7. The process comprising: extruding thermoplastic polymeric material in the form of a tubular sheet in its formative state; advancing the tubular sheet at a predetermined initial rate; quenching the tubular sheet to a temperature below its formative state; maintaining suflicient pressure within the tubular sheet to prevent collapse and to provide, when the tubular sheet is heated to a temperature Within its orientation range, an expansion to a diameter of at least 2 times the original diameter of the sheet; heating the external surface of the tubular sheet from a radiant heat source concentrically disposed around the periphery of the tubular sheet; thereafter, heating the interior surface of the tubular sheet from an internal concentrically located radiant heat source to a temperature within the orientation temperature range to expand the tubular sheet to a diameter at least 2 times its original diameter, most of the heat supplied to the tubular sheet from the external radiant heat source, said expansion being F carried out while said tubular sheet is exposed to said internal heater; advancing, while expanding, the heated tubular sheet at a rate at least 2 times the initial rate of advancement; and cooling the tubular sheet while maintaining the sheet substantially at its expanded diameter.

8. The process of claim 2 wherein the thermoplastic polymeric material is polypropylene.

9. The process comprising: extruding thermoplastic polymeric material in the form of a tubular sheet in its formative state; advancing the tubular sheet at a predetermined initial rate; quenching the tubular sheet to a temperature below its formative state; maintaining sufficient pressure within the tubular sheet to prevent collapse and to provide, when the tubular sheet is heated to a temperature within its orientation temperature range, an expansion to a diameter of at least 2 times the original diameter of the sheet; heating the exterior surface of the tubular sheet from a radiant heat source disposed around the periphery of the tubular sheet; thereafter, heating the interior surface of the tubular sheet from an internally centrally located radiant heat source to a temperature within the orientation temperature range to expand the tubular sheet to a diameter at least 2 times its original diameter, most of the heat supplied to the tubular sheet from the external radiant heat source, said expansion being carried out while said tubular sheet is exposed to said internal heater; maintaining the tubular sheet concentrically and in alignment with the exterior and interior heat sources; advancing, while expanding, the heated tubular sheet at a rate at least 2 times the intial rate of advancement; and cooling the tubular sheet while maintaining the sheet substantially at its expanded diameter.

10. The process of claim 9 wherein the thermoplastic polymeric material is polypropylene.

11. The process comprising: extruding thermoplastic polymeric material in the form of a tubular sheet in its formative state; advancing the tubular sheet at a predetermined initial rate; quenching the tubular sheet to a temperature below its formative state; maintaining sufficient pressure within the tubular sheet to prevent collapse and to provide, when the tubular sheet is heated to a temperature within its orientation temperature range, an expansion to a diameter of at least 2 times the original diameter of the sheet; heating the exterior surface of the tubular sheet to a temperature between 70% of the lowest temperature of the orientation temperature range in degrees centrigrade and just below the orientation temperature range of the polymeric material from a radiant heat source concentrically disposed around the periphery of the tubular sheet; thereafter, heating the interior surface of 'the tubular sheet from an internally disposed heat source to a temperature withinthe orientation temperature range to expand the tubular sheet to a diameter at least 2 times its original diameter, said expansion being carried out while said tubular sheet is exposed to said heat source; passing the tubular sheet over an internal guide ringe intermediate the external heat source and internal heat source to maintain the tubular sheet concentric with said internal heat source; advancing, while expanding, the heated tubular sheet at a rate at least 2 times the initial rate of advancement; and cooling the tubular sheet while maintaining the sheet substantially at its expanded diameter.

12. The process of claim 11 wherein the external radiant heat source is rotated at a rate of at least whole integer revolutions per length of unit travel of tubular sheet through the external heater length.

References Cited by the Examiner UNITED STATES PATENTS 2,141,912 7/1964 Goldman et al. 264--209 FOREIGN PATENTS 840,191 7/ 1960 Great Britain. 

1. THE PROCESS COMPRISING EXRUDING THERMOPLASTIC POLYMERIC MATERIAL IN THE FORM OF A TUBULAR SHEET IN ITS FORMATIVE STATE; ADVANCING THE TUBULAR SHEET AT A PREDETERMINED INITIAL RATE; QUENCHING THE TUBULAR SHEET TO A TEMPERATURE BELOW ITS FORMATIVE STATE; MAINTAINING SUFFICIENT PRESSURE WITHIN THE TUBULAR SHEET TO PRVENT COLLAPSE AND TO PROVIDE, WHEN THE TUBULAR SHEET IS HEATED TO A TEMPERATURE WITHIN ITS ORIENTATION TEMPERATURAE RANGE, AN EXPANSION TO A DIAMETER OF AT LEAST 2 TIMES THE ORIGINAL DIAMETER OF THE SHEET; HEATING THE TUBULAR SHEET FROM A RADIANT HEAT SOURCE TO A TEMPERATURE WITHIN THE ORIENTATION TEMPERATURE RANGAE OF THE POLYMERIC MATERIAL TO EXPAND THE TUBULAR SHEET TO A DIAMETER AT LEAST 2 TIMES ITS ORIGINAL DIAMETER; MAINTAINING THE TUBULAR SHEET CONCENTRICALLY WITH THE HEAT SOURCE; WHILE EXPANDING, THE HEATED TUBULAR SHEET AT A RATE AT LEAST 2 TIMES THE INITIAL RATE OF ADVANCEMENT; AND COOLING THE TUBULAR SHEET WHILE MAINTAINING THE SHEET SUBSTANTIALLY AT ITS EXPANDED DIAMETER. 